Thermo- and soluto-capillarity: Passive and active drops

Ryazantsev, Yuri S.; Velarde, Manuel G.; Rubio, Ramón G.; Guzmán, Eduardo; Ortega, Francisco; López, Pilar Sánchez

doi:10.1016/j.cis.2017.07.025

Cited by 29 publications

(35 citation statements)

References 113 publications

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“…Using (34)- (35), and the resulting form of C (1) and ψ (1) i,o , the inhomogeneous right-hand side of (40) include nonzero projections onto the 0-th, 1-st, and 2-nd Legendre harmonics. Accordingly, the angular component of C (2) (r, µ) is given by the first three Legendre polynomials, namely, C (2)…”

Section: Concentration Field Around the Dropletmentioning

confidence: 99%

“…n and d (2) n are unknown constant amplitudes to be determined in the matching process with the far-field boundary layer.…”

Section: Concentration Field Around the Dropletmentioning

confidence: 99%

“…In addition, equations (50)-(53) establish that self-propulsion at Ca > 0 is always accompanied by droplet deformations of order 2 with an oblate shape, ξ (2) steadily moving droplet [47], namely,…”

Section: Effect Of Deformability On the Droplet's Self-propulsionmentioning

confidence: 99%

“…Several experimental studies have recently reported self-propulsion of active droplets, whose swimming motion in viscous flows arise from spontaneously-generated surface tension gradients [1,2]. These active droplets can rely either on chemical reactions [3] or solubilization [4][5][6] as a source of chemical energy to power their self-propulsion for many hours at velocities up to one diameter per second.…”

Section: Introductionmentioning

confidence: 99%

“…In order to elucidate the mechanisms responsible for their self-propulsion, their complex individual motion and their interactions, active droplets have also attracted much theoretical and modelling effort. Unlike other currently popular microswimmers, such as bacteria or Janus particles, active droplets do not possess inherent asymmetry and, thus, rely on a symmetry-breaking instability to initiate self-propulsion [2,8,12]. In a typical scenario, the instability establishes a concentration or temperature gradient, which produces an uneven stress distribution at the droplet interface, and the droplet may self-propel due to the Marangoni effect.…”

Section: Introductionmentioning

confidence: 99%

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Self-propulsion near the onset of Marangoni instability of deformable active droplets

2018

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Experimental observations indicate that chemically active droplets suspended in a surfactantladen fluid can self-propel spontaneously. The onset of this motion is attributed to a symmetrybreaking Marangoni instability resulting from the nonlinear advective coupling of the distribution of surfactant to the hydrodynamic flow generated by Marangoni stresses at the droplet's surface. Here, we use weakly nonlinear analysis to characterize the self-propulsion near the instability threshold and the influence of the droplet's deformability. We report that in vicinity of the threshold, deformability enhances self-propulsion of viscous droplets, but hinders propulsion of drops that are roughly less viscous than the surrounding fluid. Our asymptotics further reveals that droplet deformability may alter the type of bifurcation leading to symmetry breaking: for moderately deformable droplets the onset of self-propulsion is transcritical and a regime of steady self-propulsion is stable; while in the case of highly deformable drops, no steady flows can be found within the asymptotic limit considered in this paper suggesting that the bifurcation is subcritical.

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Section: Concentration Field Around the Dropletmentioning

confidence: 99%

“…n and d (2) n are unknown constant amplitudes to be determined in the matching process with the far-field boundary layer.…”

Section: Concentration Field Around the Dropletmentioning

confidence: 99%

Section: Effect Of Deformability On the Droplet's Self-propulsionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Self-propulsion near the onset of Marangoni instability of deformable active droplets

2018

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Light‐Responsive Materials in Droplet Manipulation for Biochemical Applications

Cheng,

Kuan,

Lou

et al. 2024

Advanced Materials

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Miniaturized droplets, characterized by well‐controlled microenvironments and capability for parallel processing, have significantly advanced the studies on enzymatic evolution, molecular diagnostics, and single‐cell analysis. However, manipulation of small‐sized droplets, including moving, merging, and trapping of the targeted droplets for complex biochemical assays and subsequent analysis, is not trivial and remains technically demanding. Among various techniques, light‐driven methods stand out as a promising candidate for droplet manipulation in a facile and flexible manner, given the features of contactless interaction, high spatiotemporal resolution, and biocompatibility. This review therefore compiles an in‐depth discussion of the governing mechanisms underpinning light‐driven droplet manipulation. Besides, light‐responsive materials, representing the core of light‐matter interaction and the key character converting light into different forms of energy, are particularly assessed in this review. Recent advancements in light‐responsive materials and the most notable applications are comprehensively archived and evaluated. Continuous innovations and rational engineering of light‐responsive materials are expected to propel the development of light‐driven droplet manipulation, equip droplets with enhanced functionality, and broaden the applications of droplets for biochemical studies and routine biochemical investigations.This article is protected by copyright. All rights reserved

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Self‐Propulsion of Droplets via Light‐Stimuli Rapid Control of Their Surface Tension

Yucknovsky

Rich

Westfried

et al. 2021

Adv Materials Inter

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Such systems can mimic the motion of living cells as similar stimuli are present in nature, and can also result in our fundamental understanding of early origin of life events.The movement of droplets is linked with nonuniform surface tension (ST), resulting in fluid flow, known as the Marangoni effect. [18][19][20] This effect is based on an asymmetric exposure of the droplet surface to a chemical cue, i.e., chemotaxis, whereas ion or pH gradients are most common. [21][22][23] The asymmetry in the droplet/solution interface creates a nonuniform change in the ST. Light is a convenient tool to control the asymmetry and ST of droplets, and accordingly the motion of droplets. To use light as stimuli, there is a need for photochromic molecules next to the droplets, undergoing a photochemical process. To date, the commonly used families of photochromic molecules for mediating dynamic processes, e.g., droplet motion are azobenzenes that undergo trans-cis photoisomerization, [24,25] and spiropyran and its derivatives that overgo photocleavage primary to the proton release, resulting in minute-length reaction timescales. [26][27][28][29][30][31][32] Therefore, a yet-to-beresolved challenge in the use of such photochromic molecules is the associated timescales and especially the reversibility of the dynamic process.Herein, we propose a new approach to chemophototaxis for gaining fast subsecond responses not only to turning on the light, but also for the reversible process of turning the light off, which is based on the use of Brønsted-Lowry photoacids and photobases. This class of organic molecules undergoes a dramatic change in their dissociation equilibrium constant (pKa) upon light excitation. [33] Thus, only in their electronically excited state do photoacids and photobases behave as strong acids and bases. The above-mentioned spiropyrans are also referred to as photoacids since their photocleavage process from spiropyran to merocyanine involves the release of a proton, however, there is a large difference in terms of mechanism and timescales relative to Brønsted-Lowry photoacids. In this context, we hypothesize that the fast excited-state proton transfer (ESPT) of Brønsted-Lowry photoacids and photobases can induce rapid chemical changes on the surface of pH-sensitive droplets and initiate their motion. We show here a variety of different droplet systems involving the use of photoacids and photobases either within the droplet, on its surface, or in solution, whereas we use light to self-propel and to guide the movement of the droplet. Nature demonstrates many examples of response and adaptation to external stimuli. Here, this study focuses on self-propulsion (motion) while presenting several self-propelling droplet systems responsive to pH gradients. Light is used as the gating source to gain reversibility, avoid the formation of chemical wastes, and control the self-propulsion remotely. To achieve light-stimuli ultrafast response, photoacids and photobases are used, capable of donating or capturing a proton, respect...

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Thermo- and soluto-capillarity: Passive and active drops

Cited by 29 publications

References 113 publications

Self-propulsion near the onset of Marangoni instability of deformable active droplets

Self-propulsion near the onset of Marangoni instability of deformable active droplets

Light‐Responsive Materials in Droplet Manipulation for Biochemical Applications

Self‐Propulsion of Droplets via Light‐Stimuli Rapid Control of Their Surface Tension

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